JPH077435A - Dither mixing circuit of ΣΔ AD converter - Google Patents

Abstract

(57) [Abstract] [Object] Regarding a dither mixing circuit in a ΣΔ AD converter, an object of the present invention is to provide a dither mixing circuit of the ΣΔ AD converter in which dither can be added without changing the input signal. . A ΣΔ modulator 2 for charging a capacitor C with a voltage difference between a voltage of a signal source 1 and a reference voltage and integrating a voltage difference between the charged voltage and a reference voltage to generate an output, In a ΣΔ AD converter configured to perform 1-bit AD conversion using this integrated output, a dither mixing circuit 3 is provided, and dither is mixed in the voltage of the signal source 1 by changing the value of the reference voltage during integration. Then, S of the ΣΔ AD converter
It is configured to prevent the level dependency of the / N characteristic.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a .SIGMA..DELTA. AD converter, and more particularly to a dither mixing circuit of a .SIGMA..DELTA. AD converter which can add dither without changing an input signal.

The S / N characteristic of the ΣΔ AD converter changes depending on the level of the input signal. On the other hand, a method is known in which a minute signal (dither) is mixed into an input signal to prevent such level dependence.

The dither mixing circuit in the ΣΔ AD converter is required to be capable of mixing dither without changing the input signal.

[0004]

2. Description of the Related Art FIG. 4 shows a ΣΔ AD converter having a dither signal source, where 11 is a signal source and 12 is ΣΔ.
An AD converter, 13 is a digital filter. The ΣΔ AD converter 12 performs 1-bit analog-to-digital (AD) conversion on the analog input signal of the signal source 11 at a certain sampling frequency, and limits the output of this output through a digital filter 13 to obtain a digital signal. Get the converted output.

Since the S / N characteristic of the ΣΔ AD converter changes depending on the level of the input signal, this can be improved by mixing an appropriate minute signal (dither) into the input signal. In FIG. 4, reference numeral 14 denotes a dither signal source, which supplies a dither signal composed of an appropriate DC signal or AC signal (square wave or the like) to the signal source 1 via an adding unit 15.
By adding it to the input signal of 1, it is possible to prevent the S / N characteristic of the ΣΔ AD converter 12 from depending on the level.

FIG. 5 shows an example of a dither mixing circuit and a ΣΔ modulation section in a conventional ΣΔ AD converter. Two
Reference numeral 1 denotes a ΣΔ modulation unit, SW ₁ to SW ₄ are switches forming a switched capacitor unit, C ₀ is a sampling capacitor, OP ₁ is an operational amplifier (operational amplifier) forming an integrator, C ₁ is the integrating capacitor. Reference numeral 22 is a dither mixing circuit, OP ₂ is an operational amplifier (operational amplifier), and R _{1 and} R ₂ are resistors.

In the ΣΔ modulator 21, the switched key
Switch SW of capacitor₁ And SW ₂ , And switch
SW₃ And SW_Four Are alternately turned on at a certain sampling frequency.
Turn on and off repeatedly.

When the switches SW ₁ and SW ₂ are on, the switches SW ₃ and SW ₄ are off, and the capacitor C ₀ is charged by the voltage difference between the input signal and the reference voltage V _ref . When the switches SW ₃ and SW ₄ are on, the switches SW ₁ and SW ₂ are off, and the integrating capacitor C ₁ is integrated by the difference voltage between the reference voltage V _ref and the voltage of the capacitor C ₀ .

When the voltage of the signal source 11 is V _S , the operational amplifier OP ₂ in the dither mixing circuit 22 generates an output obtained by adding − (R ₂ / R ₁ ) V _S centered on the input of the + terminal. . At this time, dither can be mixed in the ΣΔ AD converter by adding a certain voltage α (V) to the reference voltage V _ref as an input to the + terminal.

FIG. 6 shows another example of the dither mixing circuit and the ΣΔ modulation section in the conventional ΣΔ AD converter. In FIG. 6, the ΣΔ modulator 21 is the same as that in the case of FIG. Reference numeral 23 is a dither mixing circuit, and 24 is a dither signal source. In the dither mixing circuit 23, SW _{11 to} SW ₁₄ 4
Is a switch that constitutes the second switched capacitor unit, and C ₀₁ is a sampling capacitor.

In the dither mixing circuit 23, the switches SW ₁₁ and SW ₁₂ and the switches SW ₁₃ and SW _{14 in} the switched capacitor section are switched in the ΣΔ modulation section 21.
Alternately turned on in synchronization with the switch SW ₁ and SW _2, and the switch SW ₃ and SW ₄ of the capacitor unit, repeatedly off, the electric charge charged from the dither signal source 24 to the capacitor C ₀₁ is the integral of the ΣΔ modulation unit 21 Dither mixing is performed by charging the capacitor C ₁ .

[0012]

ΣΔA shown in FIG.
The D converter is provided with a dither mixing circuit including an operational amplifier or the like in the preceding stage to add dither to the input signal. In this circuit, since the input signal passes through the operational amplifier placed in the previous stage, there is a problem that the characteristics of the operational amplifier are easily affected.

Further, in the ΣΔ AD converter shown in FIG. 6,
Since an operational amplifier or the like is not required before the ΣΔ AD converter to add dither, it is not affected by the characteristics of the operational amplifier, but since it has two switched capacitor sections in the signal input section, both switched It is necessary to strictly adjust the operation timing of the capacitor unit. Further, there is a problem that the accuracy of the capacitor of the switched capacitor unit on the dither mixed side also causes the characteristic deterioration of the ΣΔ AD converter.

The present invention is intended to solve the problems of the prior art as described above.
Since no operational amplifier or the like is required in the preceding stage of the ΔAD converter, the characteristics of the operational amplifier are not affected and the same dither signal can be always applied, so that the deterioration of the S / N characteristic due to the change of the dither signal is prevented. It is an object of the present invention to provide a ΣΔ AD converter capable of performing the above.

[0015]

FIG. 1 shows the basic configuration of the present invention. (1) ΣΔAD provided with a ΣΔ modulator 2 that charges the capacitor C with a voltage difference between the voltage of the signal source 1 and the reference voltage, and integrates the voltage difference between the charging voltage and the reference voltage to generate an output
In the converter, the dither mixing circuit 3 that adds the dither to the reference voltage and outputs it is provided, and the dither is mixed into the voltage of the signal source 1 by changing the reference voltage at the time of integration.

(2) In (1), the dither mixing circuit 3 has an operational amplifier OP whose negative-phase input is grounded and whose positive-phase input is supplied with a reference voltage. The output of this operational amplifier OP is integrated. It should be given as a reference voltage.

(3) In (2), the operational amplifier OP has two sets of feedback circuits each consisting of a series circuit of a resistor and a switch having different values between the output terminal and the anti-phase input, and the switches are alternately turned on. Control is performed so that a voltage obtained by adding a square wave to the reference voltage is applied as the reference voltage for integration.

(4) In (2), the operational amplifier OP has a plurality of sets of feedback circuits consisting of series circuits of resistors and switches having different values between the output terminal and the anti-phase input, and one of the switches is By controlling to be on, a voltage obtained by adding a different voltage to the reference voltage is given as the reference voltage for integration.

[0019]

(1) In the ΣΔ modulation section 2, the capacitor C is charged with the voltage difference between the voltage of the signal source 1 and the reference voltage, and the voltage difference between the charging voltage and the reference voltage is integrated to obtain an output. Occur. The ΣΔ AD converter uses this integrated output to generate a 1-bit AD conversion output.

In this case, by providing the dither mixing circuit 3,
The dither is added to the reference voltage and output, and the reference voltage at the time of integration in the ΣΔ modulation unit 2 is changed so that the dither is mixed in the voltage of the signal source 1. No operational amplifier or the like is required in the preceding stage of the converter, and therefore the performance of the ΣΔ AD converter is not affected by the characteristics of the operational amplifier.

(2) In the case of (1), the dither mixing circuit 3 is provided with an operational amplifier OP whose negative-phase input is grounded and whose positive-phase input is supplied with a reference voltage. It is possible to add dither to the input signal by applying it as the reference voltage.

In the cases of (3) and (2), two sets of feedback circuits in which a series circuit of a resistor and a switch having different values are connected between the output terminal and the anti-phase input are provided in the operational amplifier OP, and each switch is provided. By alternately turning on the control, a voltage obtained by adding a square wave to the reference voltage can be given as a reference voltage for integration, and thus a square wave dither can be given to the input signal.

In the cases of (4) and (2), the operational amplifier OP is provided with a plurality of sets of feedback circuits each including a series circuit of a resistor and a switch having different values between the output terminal and the negative phase input, By controlling the switch to be turned on, a voltage obtained by adding a different voltage to the reference voltage can be given as the reference voltage at the time of integration, whereby a different DC voltage can be given to the input signal as dither.

[0024]

FIG. 2 shows an embodiment of the present invention in which a DC voltage is used as dither. The same parts as in FIG. 5 are indicated by the same symbols, and 25 is a dither mixing circuit. In the dither mixing circuit 25,
OP ₃ is an operational amplifier (operational amplifier), and R ₁ and R ₂ are resistors.

An input signal is given from the signal source 11 to point A.
Be done. In the switched capacitor part of the ΣΔ modulator 21
Switch SW₁ , SW₂ Switch is on
SW ₃ , SW_Four Is off, the input voltage Vs and the reference voltage are
Pressure V_refDepending on the voltage difference between₀ Is charged
To be done.

Next, in the switched capacitor section,
When the switches SW ₃ and SW ₄ are on, the switch SW
_{When 1} and SW ₂ are off, the voltage of the difference between the charging voltage of the capacitor C ₀ and the reference voltage V _{ref2 at the} point B is integrated into the integrating capacitor C ₁ .

The reference voltage V _ref2 is + of the operational amplifier OP ₃ .
A DC dither voltage determined by the ratio of the resistors R _{1 and} R ₂ is applied to the reference voltage V _{ref at the} input.

As a result, the voltage integrated in the integrating capacitor C ₁ is based on the voltage obtained by adding dither to the input signal, so that the input signal can be input to the ΣΔ AD converter without passing through an operational amplifier or the like. You can

FIG. 3 shows another embodiment of the present invention, in which a square wave voltage or a different DC voltage is used as the dither. In the dither mixing circuit 26, the same parts as those in FIG.
₅ and SW ₆ are switches, and R ₃ (R ₃ ≠ R ₂ ) is a resistor.

In the embodiment of FIG. 3, the switch SW
_By alternately turning on and off ₅ , ₅ and SW ₆ , the reference voltage V _ref3 to which the reference voltage V _ref is dithered and which is a square wave in which the reference voltage V _ref is changed by a small voltage is _applied to the operational amplifier O.
The output of P ₃ is obtained at the point B, and this voltage integrates the charge in the integrating capacitor C ₁ as in the case of FIG.

Also, in the embodiment of FIG. 3, the switch S
By turning on either W ₅ or SW ₆ , it is possible to obtain the reference voltage V _ref3 to which the dither having different DC voltages is applied at the point B, so that the dither in the ΣΔ AD converter is arbitrarily changed. be able to.

[0032]

As described above, according to the present invention, ΣΔAD can be obtained without passing an input signal through an operational amplifier or the like.
Since it can be input to the converter, it is possible to prevent the deterioration of the S / N characteristic due to the influence of the operational amplifier or the like.

Further, according to the present invention, the value of the dither signal can be changed only by changing the resistance of the operational amplifier circuit in the dither mixing circuit.

Further, according to the present invention, since only the portion of the switched capacitor portion of the ΣΔ modulation portion that provides the reference voltage is changed, it can be easily applied to the ΣΔ AD converter of the differential circuit. The same effect can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a principle configuration of the present invention.

FIG. 2 is a diagram showing an embodiment of the present invention.

FIG. 3 is a diagram showing another embodiment of the present invention.

FIG. 4 is a diagram showing a ΣΔ AD converter having a dither signal source.

FIG. 5 is a diagram showing an example of a dither mixing circuit and a ΣΔ modulation section in a conventional ΣΔ AD converter.

FIG. 6 is a diagram showing another example of a dither mixing circuit and a ΣΔ modulation section in a conventional ΣΔ AD converter.

[Explanation of symbols]

 1 Signal Source 2 ΣΔ Modulator 3 Dither Mixing Circuit C Capacitor OP Operational Amplifier

Claims (4)

[Claims] 1. A capacitor (C) is charged with a voltage which is a difference between a voltage of a signal source (1) and a reference voltage, and the voltage which is a difference between the charged voltage and the reference voltage is integrated to generate an output. In a ΣΔ AD converter including a ΣΔ modulator (2), a dither mixing circuit (3) that adds a dither to a reference voltage and outputs the added dither is provided, and by changing the reference voltage during the integration, the signal source (1 ) The dither mixing circuit of the ΣΔ AD converter, wherein the dither is mixed into the voltage. 2. The dither mixing circuit (3) has an operational amplifier (OP) whose negative-phase input is grounded and whose positive-phase input is given a reference voltage, and the output of the operational amplifier (OP) is integrated by the integration. The dither mixing circuit for the ΣΔ AD converter according to claim 1, wherein the dither mixing circuit is provided as a reference voltage for the time. 3. The operational amplifier (OP) has two sets of feedback circuits each consisting of a series circuit of a resistor and a switch having different values between the output terminal and the negative phase input, and each switch is alternately turned on. The dither mixing circuit of the ΣΔ AD converter according to claim 2, wherein a voltage obtained by adding a square wave to the reference voltage is applied as the reference voltage during the integration. 4. The operational amplifier (OP) has a plurality of sets of feedback circuits each including a series circuit of a resistor and a switch having different values between the output terminal and the negative phase input, and turns on any one of the switches. The dither mixing circuit of the ΣΔ AD converter according to claim 2, wherein a voltage obtained by adding a different voltage to the reference voltage is given as a reference voltage during the integration by controlling.